1. Field of the Invention
The present invention relates to fluidics apparatus and substrates for fluidics apparatus, and uses of such apparatus and substrates. Of particular, but not necessarily exclusive, interest is fluid sample manipulation in a microfluidics context. The invention has particular, but not exclusive, application to the manipulation of liquid droplets, for example in biological, biochemical, medical, veterinary and chemical assays, analysis, diagnosis, and synthesis and production of reagents and chemicals.
The present invention further relates to methods for lysing cells and to the use of a fluidics apparatus for lysing cells in a fluid sample.
2. Related Art
Microfluidics devices are well known for handling and analysing small volumes of fluids. For example, WO 2005/100953 discloses a system for measuring viscosity of fluids. Fluids are moved along microfluidic passageways using a thermal pump.
Alternative approaches to microfluidics liquid handling include the use of surface acoustic wave devices, as described in US 2007/0140041. In that document, there is disclosed the problem of mixing two microfluidics streams at a manifold, since at microfluidics dimensions, some liquids flow via laminar flow, and the lack of turbulence makes mixing difficult. Accordingly, US 2007/0140041 seeks to improve mixing between two fluid flows at a microfluidics manifold using surface acoustic waves (SAWs). A SAW transducer is located in contact with the manifold in order to promote mixing of the fluid streams at the manifold junction.
Surface acoustic waves (SAWs, the most common being Rayleigh waves) are acoustic waves that can be caused to travel along the surface of a material. Surface acoustic waves can be conveniently formed at the surface of a piezoelectric material by the application of a suitable electrical signal to an electrode arrangement at the surface of the piezoelectric material. A suitable electrode arrangement utilizes interdigitated electrodes, where a first electrode has an arrangement of parallel electrode fingers having a regular spacing between the fingers. A corresponding second electrode of similar shape has fingers which protrude into the gaps between the fingers of the first electrode. The combination of the electrode arrangement and the piezoelectric material forms a transducer.
SAW transducers are known particularly for use in frequency filters in telecommunications devices such as mobile telephones. In such a filter, there is an input transducer and an output transducer. The input signal is applied to the input transducer, to form a series of SAWs which propagate to the output transducer. At the output transducer, the SAWs are converted back into an electrical signal. For example, Dogheche et al [E. Dogheche, V. Sadaune, X. Lansiaux, D. Remiens, and T. Gryba “Thick LiNbO3 layers on diamond-coated silicon for surface acoustic wave filters” Applied Physics Letters Vol. 81, No. 7 (12 Aug. 2002) p. 1329] disclose the fabrication of piezoelectric films for SAW filters. Typically, such filters are formed using known piezoelectric substrates such as quartz, LiTaO3 or LiNbO3. However, the formation of suitable interdigitated electrode patterns on the surface of such substrates by conventional photolithography whilst providing a filter operable up to suitable telecommunications frequencies is difficult. Accordingly, Dogheche et al formed thick (around 1 μm thick) piezoelectric LiNbO3 layers on diamond-coated silicon and demonstrated their operation as SAW filters at 293 MHz.
It has also been noted that it is possible to provide quasi crystalline structures in order to manipulate SAWs. It has been shown to be possible to use a variety of phononic bandgap structures to affect an acoustic wavefront generated in a piezoelectric material. For example, Wu el at [Wu, T. T., Z. G. Huang, and S. Y. Liu, “Surface acoustic wave band gaps in micro-machined air/silicon phononic structures—theoretical calculation and experiment” Zeitschrift Fur Kristallographie, 2005. 220(9-10): p. 841-847] discuss their investigations of the phononic band gaps in structures formed by micromachining silicon with a square lattice arrangement of holes. The transducer was formed with interdigitated electrodes having parallel fingers. Furthermore, Wu et al [Wu, T. T., L. C. Wu, and Z. G. Huang, “Frequency band-gap measurement of two-dimensional air/silicon phononic crystals using layered slanted finger interdigital transducers” Journal of Applied Physics, 2005. 97(9): p. 7] disclose the results of investigations using a similar phononic crystal using electrodes with interdigitated non-parallel fingers in the form of a fan shape. Furthermore, in a purely theoretical paper, Kuo and Ye [Kuo, C. H. and Z. Ye, “Sonic crystal lenses that obey the lensmaker's formula” Journal of Physics D-Applied Physics, 2004. 37(15): p. 2155-2159] discuss the properties of structures that could be used to focus acoustic waves.
The term “phononic crystal” is used as an analogy to a “photonic crystal”. In a photonic crystal, a periodic structure causes reflections due to scattering of incident light, thereby allowing interference between the reflected light and the incident light as it propagates through the “crystal” (which typically is formed of an arrangement of dielectric materials based on a regular array, such as a Bragg reflector), at one or more wavelengths and angles of incidence. This interference manifests itself as a prevention of propagation of the light through the crystal at a certain wavelength (or range of wavelengths) and direction. Thus, there is a “band gap” of frequencies at which light cannot propagate through the photonic crystal. A phononic crystal, by analogy, has a periodic arrangement of discontinuities or variations in the mechanical properties of the material or materials making up the phononic crystal. Such a phononic crystal can prevent acoustic or mechanical waves of specific wavelength from propagating through the crystal. Since SAWs can be formed at tightly defined frequencies, the effect of phononic crystals on the propagation of SAWs has been studied by several groups.
Mohammadi et al [Mohammadi, S., et al., “Complete phononic bandgaps and bandgap maps in two-dimensional silicon phononic crystal plates” Electronics Letters, 2007. 43(16): p. 898-899] disclose the formation of complete phononic band gap structures using a square array of holes or a hexagonal array of holes in a silicon plate. In a publication from the same group, Mohammadi et al [Mohammadi, S., et al., “Evidence of large high frequency complete phononic band gaps in silicon phononic crystal plates” Applied Physics Letters, 2008. 92(22): p. 3] discuss the formation of large complete phononic band gaps using a hexagonal array of holes through a silicon plate.
Olsson et al [Olsson, R. H., et al., “Microfabricated VHF acoustic crystals and waveguides” Sensors and Actuators a-Physical, 2008. 145: p. 87-93] disclose the formation of acoustic bandgaps in a structure formed by including periodic arrays of tungsten scatterers in a silica matrix. Waveguides for the acoustic waves are provided by removing selected scatterers along a desired path.
Vasseur et al [Vasseur, J. O. et al., 2008. Absolute forbidden bands and waveguiding in two-dimensional phononic crystal plates. Physical Review B (Condensed Matter and Materials Physics), 77(8), 085415-15] set out a study of phononic bandgaps in a two dimensional phononic crystal plate formed by arrays of cylinders of a first material in a plate of a second material.
US 2008/0211602 discloses an acoustic wave device with a piezoelectric layer with transducer electrodes formed over a substrate, there being an omnidirectional acoustic mirror formed between the piezoelectric layer and the substrate.
Other workers have used SAWs in the manipulation of liquids. For example, Renaudin et al [A. Renaudin, P. Tabourier, V. Zhang, J. C. Camart and C. Druon “SAW nanopump for handling droplets in view of biological applications” Sensors and Actuators B, 113, 2006, p. 389] report on the fabrication and development of a SAW device for microfluidics for biological applications. SAWs at about 20 MHz are generated by interdigitated electrode transducers laid on a LiNbO3 piezoelectric substrate. Droplets are transported along the surface of the transducer where hydrophilic micro tracks are provided between hydrophobic areas. Furthermore, the same research group [Renaudin, A. et al., 2009. Monitoring SAW-actuated microdroplets in view of biological applications. Sensors and Actuators B: Chemical, 138(1), 374-382] set out a method for determining the position of the droplet using echo signals detected by interdigitated transducers.
Du et al [Du, X. Y. et al., 2009. Microfluidic pumps employing surface acoustic waves generated in ZnO thin films. Journal of Applied Physics, 105(2), 024508-7] propose using ZnO thin films on Si substrates to form surface acoustic wave operated microfluidic pumps.
Frommelt et al [Frommelt, T. et al., 2008. Flow patterns and transport in Rayleigh surface acoustic wave streaming: combined finite element method and raytracing numerics versus experiments. Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions on, 55(10), 2298-2305] investigate the patterns of liquid flow and particle transport inside a droplet subjected to surface acoustic waves.
Shi et al [Shi, J. et al., 2008. Focusing microparticles in a microfluidic channel with standing surface acoustic waves (SSAW). Lab on a Chip, 8(2), 221-223] propose using opposed interdigitated transducers to form an aligned arrangement of beads moving along a channel.
Wu and Chang [Wu, T. & Chang, I., 2005. Actuating and detecting of microdroplet using slanted finger interdigital transducers. Journal of Applied Physics, 98(2), 024903-7] disclose the movement of droplets on a SAW substrate by control of the signal applied to interdigitated transducers having fingers arranged in a slanting configuration.
Tan et al [Tan, M. K., J. R. Friend, and L. Y. Yeo, “Microparticle collection and concentration via a miniature surface acoustic wave device” Lab on a Chip, 2007. 7(5): p. 618-625] disclose the use of SAWs to collect microparticles such as pollen particles in a droplet of water. A water droplet is conveyed along a SAW transducer via a fluidic track.
Concentration of microparticles in droplets by asymmetric application of surface acoustic waves has also been described. Techniques described for breaking the symmetry of a surface acoustic wave involve aligning a drop on the edge of a parallel electrode interdigital transducer [A. Zhang, W. Liu, Z. Jiang and J. Fei, Appl. Acoust., 2009, 70, 1137-1142.], positioning a gel to partially absorb the surface acoustic wave reflection (so that only part of the drop lies in the transmission pathway) [H. Li, J. R. Friend and L. Y. Yeo, Biomed. Microdev., 2007, 9, 647-656], or using a more complex IDT that focuses the surface acoustic wave [R Shilton, M. Tan and L. Yeo, and J. Friend, J. Appl. Phys., 2008, 104, 014910] using circular transducers with a fixed frequency and excitation pathway.
Bennes et al [J. Bennes, S Alzuage, F. Chemoux, S. Ballandras, P. Vairac, J-F Manceau and F. Bastien, “Detection and high-precision positioning of liquid droplets using SAW systems” IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, 2007, 54(10): p. 2146-2151] disclose droplet detection and positioning using SAWs. The SAW devices used are formed from lithium niobate substrates (LiNbO3 cut (XY1)/128°). Bennes et al explain that the droplets are moved due to the refraction of incoming SAWs along the substrate surface at the air/liquid interface, producing a resultant force which can have a component directed along the substrate surface. The LiNbO3 substrate is treated to make it hydrophobic—this increases the contact angle with an aqueous droplet and decreases the force required to move the droplet by interaction with SAWs.
WO 02071051 discloses acoustic ejection of biomolecular samples for mass spectrometry.
WO 2007/128045 discloses the use of a SAW transducer to atomize a liquid droplet from a substrate coupled to a piezoelectric transducer by a fluid coupling layer, thereby forming zeolite nanocrystals.
Fluidics systems may be useful in the analysis of biological samples, for example in point-of-care diagnostic applications and portable biosensors. However, biological samples present a particular challenge for sample manipulation and analysis in fluidics, particularly microfluidics. Preparation of biological samples is often complex, involving multiple steps. Notably, for a biological sample containing cells the molecule of interest may be an intracellular molecule, such that sample preparation requires a cell disruption step in order to render intracellular molecules accessible for analysis and applications such as immunodiagnostics and pathogen detection.
There are a variety of ways to disrupt cells in order to release intracellular molecules for analysis. Cells are enclosed by a lipid bilayer called the plasma membrane (also known as the cell membrane, or cytoplasmic membrane), which defines the boundaries of the cell. Cell disruption by rupture of the plasma membrane is termed cell lysis, and this can be achieved by a variety of chemical and physical methods.
A typical chemical lysis procedure involves numerous steps, including the addition of lytic agents (e.g. enzymes, detergents), washing (usually using centrifugation steps), and elution of the processed samples for further analysis. Physical lysis procedures include heating and mechanical methods such as agitation with small particles (e.g. glass beads) and sonication (or ultrasonication). Sonication typically involves transmitting mechanical energy, via an immersed probe that oscillates with high frequency, to a solution containing cells in suspension, and resultant cavitation (the creation and collapse of microscopic bubbles) ruptures cells in the sample.
Chemical cell lysis procedures have been integrated into microfluidic systems [P. Sethu, M. Anahtar, L. L. Moldawer, R. G. Tompkins, and M. Toner, Continuous Flow Microfluidic Device for Rapid Erythrocyte Lysis, Anal. Chem. 2004, 76, 6247-6253; X. Chen, D. F. Cui and C. C. Liu, On-line cell lysis and DNA extraction on a microfluidic biochip fabricated by microelectromechanical system technology, Electrophoresis 2008, 29, 1844-1851]. However, these methods require lytic agents, which may significantly dilute the molecule of interest and thereby compromise sensitivity of subsequent detection steps. These methods also require a cumbersome liquid-driving system to move the liquids around the chip, which is impractical for point-of-care applications. Removal of lytic and/or eluting agents may be required for downstream processing or analysis of the sample, for example because these agents inhibit reactions (e.g. PCR-based amplification of nucleic acids), or because they compromise the molecule of interest.
Techniques have been developed for chemical-free lysis of cells in samples on microfluidic platforms. These include heating [S. Baek, J. Min and J.-H. Park, Wireless induction heating in a microfluidic device for cell lysis, Lab on a Chip, 2010, 10, 909-917], applying an electric field [D. W. Lee, Y.-H. Cho, A continuous electrical cell lysis device using a low dc voltage for a cell transport and rupture, Sensors and Actuators B, 2007, 124, 84-89], or using mechanical forces to disrupt the cells by the combined action of magnetic fields [J. Siegrist, R. Gorkin, M. Bastien, G. Stewart, R. Peytavi, H. Kido, M. Bergeron and M. Madou, Validation of a centrifugal microfluidic sample lysis and homogenization platform for nucleic acid extraction with clinical samples, Lab on a Chip, 2010, 10, 363-371], by using filter structures [D. Di Carlo, K.-H. Jeong and L. P. Lee, Reagentless mechanical cell lysis by nanoscale barbs in microchannels for sample preparation, Lab on a Chip, 2003, 3, 287-291] or by ultrasonication [M. T. Taylor, P. Belgrader, B. J. Furman, F. Pourahmadi, G. T. A. Kovacs and M. A. Northrup, Lysing Bacterial Spores by Sonication through a Flexible Interface in a Microfluidic System, Analytical Chemistry 2001, 73, 492-496 and M. T. Taylor, Apparatus and method for rapid disruption of cells or viruses, WO03055976 (Cepheid, Inc.)].
However, heat, electric fields or cavitation may compromise molecules of interest. Electrical lysis may be integrated in a microfluidics chip with other functions [J. Cheng, E. L. Sheldon, L. Wu, A. Uribe, L. O. Gerrue, J. Carrino, M. J. Heller, J. P. O'Connell, Preparation and hybridization analysis of DNA/RNA from E. coli on microfabricated bioelectronic chips, Nature Biotechnology, 1998, 16, 541-546], but other physical lysis methods require the addition of external actuations into the system to move the fluids around the chip, in a similar fashion as chemical-based lysis platforms. This has been a particular difficulty hindering the development of fully integrated “sample-to-answer” solutions for molecular diagnostics [P. Yager, T. Edwards, E. Fu, K Helton, K. Nelson, M. R. Tam and B. H. Weigl, Microfluidic diagnostic technologies for global public health, Nature, 2006, 442, 412-418].